1. Field of the Invention
The invention relates to the regeneration of fluidized catalytic cracking catalyst.
2. Description of Related Art
Catalytic cracking of hydrocarbons is carried out in the absence of externally supplied H2, in contrast to hydrocracking, in which H2 is added during the cracking step. An inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator In the fluidized catalytic cracking (FCC) process, hydrocarbon feed contacts catalyst in a reactor at 425.degree. C.-600.degree. C., usually 460.degree. C.-560 C. The hydrocarbons crack, and deposit carbonaceous hydrocarbons or coke on the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, and is then regenerated. In the catalyst regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Coke burns off, restoring catalyst activity and simultaneously heating the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Most older FCC units regenerate the spent catalyst in a single dense phase fluidized bed of catalyst. Although there are myriad individual variations, typical designs are shown in U.S. Pat. No. 3,849,291 (Owen) and U.S. Pat. No. 3,894,934 (Owen et al), and U.S. Pat. No. 4,368,114 (Chester et at.) which are incorporated herein by reference.
Most new units are of the High Efficiency Regenerator (H.E.R.) design using a coke combustor, a dilute phase transport riser, and a second dense bed, with recycle of some hot, regenerated catalyst from the second dense bed to the coke combustor. Units of this type are shown in U.S. Pat. No. 3,926,778 (which is incorporated by reference) and many other recent patents. The H.E.R. design is used in most new units because it permits operation of an FCC with less catalyst inventory (and hence less catalyst loss and lower catalyst makeup), and because such units tend to have both less CO emissions and less NOx emissions than the single dense bed regenerators.
Unfortunately, it has not been economically justifiable to convert older style, single dense bed regenerators to the modern H.E.R. design because of the high capital cost associated with simply scrapping the old single bed regenerator. Attempts to simply use the old single stage regenerator as part of a modern two stage, H.E.R. design have not been too successful, as the old single stage units are much larger than either of the beds in an H.E.R. unit. Another complication has been that many of the older units were not designed to operate at the higher temperatures associated with complete CO combustion.
Rather than scrap older FCC regenerators, refiners have tried to improve them, and the FCC process, as much as possible with improvements in catalyst and catalyst additives.
Most FCC units now use zeolite-containing catalyst having high activity and selectivity. These catalysts work best when the amount of coke on the catalyst after regeneration is relatively low. It is desirable to regenerate zeolite catalysts to as low a residual carbon level as is possible. It is also desirable to burn CO completely within the catalyst regenerator system to conserve heat and to minimize air pollution. Heat conservation is especially important when the concentration of coke on the spent catalyst is relatively low as a result of high catalyst selectivity. Among the ways suggested to decrease the amount of carbon on regenerated catalyst and to burn CO in the regenerator is to add a CO combustion promoter metal to the catalyst or to the regenerator. Metals have been added as an integral component of the cracking catalyst and as a component of a discrete particulate additive, in which the active metal is associated with a support other than the catalyst. U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121, incorporated herein by reference, introduced relatively large-sized particles containing CO combustion-promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, of small-sized catalyst particles, are cycled between the cracking reactor and the catalyst regenerator, while the combustion-promoting particles remain in the regenerator. Oxidation-promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc. impregnated on an inorganic oxide such as alumina, are disclosed.
U.S. Pat. No. 4,072,600 and U.S. Pat. No. 4,093,535 teach combustion-promoting metals such as Pt, Pd, Ir, Rh, Os, Ru and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory.
Many FCC units use CO combustion promoters. This reduces CO emissions, but usually increases nitrogen oxides (NOx) in the regenerator flue gas. It is difficult in a catalyst regenerator to completely burn coke and CO in the regenerator without increasing the NOx content of the regenerator flue gas.
Although many refiners have recognized the problem of NOx emissions from FCC regenerators, the solutions proposed have not been completely satisfactory. The approaches taken so far have generally been directed to special catalysts which will inhibit the formation of NOx in the FCC regenerator, or to process changes which reduce NOx emissions from the regenerator.
Recent catalyst patents include U.S. Pat. No. 4,300,997 and its division U.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO-combustion promoter The bi-metallic CO combustion promoter is reported to do an adequate job of converting CO to CO2, while minimizing the formation of NOx.
Another catalyst development is disclosed in U.S. Pat. No. 4,199,435 which suggests steam treating conventional metallic CO combustion promoter to decrease NOx formation without impairing too much the CO combustion activity of the promoter.
Process modifications are suggested in U.S. Pat. No. 4,413,573 and U.S. Pat. No. 4,325,833 directed to two-and three-stage FCC regenerators, which reduce NOx emissions.
U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCC catalyst, without backmixing, to minimize NOx emissions.
U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel to the upper portion of a FCC regenerator to minimize NOx emissions. Oxides of nitrogen formed in the lower portion of the regenerator are reduced in the reducing atmosphere generated by burning fuel in the upper portion of the regenerator.
U.S. Pat. No. 4,235,704 suggests that too much CO combustion promoter causes NOx formation, and calls for monitoring the NOx content of the flue gases, and adjusting the concentration of CO combustion promoter in the regenerator based on the amount of NOx in the flue gas.
The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volume of flue gas by using oxygen rather than air in the FCC regenerator, with consequent reduction in the amount of flue gas produced.
All the catalyst and process patents discussed above from U.S. Pat. No. 4,300,997 to U.S. Pat. No. 4,542,114, are incorporated herein by reference.
None of the approaches described above provides the perfect solution. Various catalytic approaches, e.g., use of bi-metallic CO combustion promoters, provide some assistance, but the cost and complexity of a bi-metallic combustion promoter is necessary. The reduction in NOx emissions achieved by catalytic approaches helps some but still may fail to meet the ever more stringent NOx emissions limits set by local governing bodies. Much of the NOx formed is not the result of combustion of N2 within the FCC regenerator, but rather combustion of nitrogen-containing compounds in the coke entering the FCC regenerator. Bi-metallic combustion promoters are probably best at minimizing NOx formation from N2.
We have discovered a way to overcome many of the deficiencies of the prior art methods of regenerating spent FCC catalysts in a single dense bed by designing the regenerator so that whenever high oxygen concentrations are present, there will also be relatively high concentrations of carbonaceous particles, and usually high concentrations of CO. The presence of carbonaceous particles and/or a generally reducing atmosphere, are known to reduce the formation of NOx. Although this phenomenon is generally known, it has never been used effectively in commercial FCC regenerators, because of the difficulty of designing such a regenerator. We found a way to achieve something approaching co-current regeneration, wherein oxygen concentration can be matched to nitrogenous coke concentration, and wherein the chance of localized high temperature regions in the regenerator is greatly reduced or eliminated. This approach can best be followed by first considering an "ideal" but impractical regenerator, then the regenerator of the present invention which comes close to achieving an "ideal" regeneration, as far as NOx emissions and heat recovery are concerned.
An "ideal" but unusable regenerator would be a 500 to 1000' foot long tube, with an inlet at the base for coked catalyst and air or other gas and an outlet at the top for regenerated catalyst and flue gas. The geometry of the tube would be selected, relative to gas flow, so that dilute phase, turbulent flow would be maintained.
This is an ideal regenerator as far as NOx and heat recovery are concerned High oxygen concentrations, which would normally rapidly oxidize nitrogenous coke and tend to form NOx are balanced or offset by high coke concentrations. The presence of coke and formation of CO (from coke combustion) both suppress NOx formation or promote its reduction to nitrogen.
In the ideal regenerator there is always good contact of catalyst and flue gas, so that as things heated up, either from coke combustion, or from afterburning of CO to CO2, the catalyst particles function as efficient heat sinks. Turbulent plug flow prevents localized high concentrations of oxygen, which could give localized high temperatures and/or localized high NOx concentrations.
Use of near stoichiometric air would mean that the oxygen concentration would decrease simultaneously with coke on catalyst. Coke concentration and oxygen concentration would both decrease asymptotically. The reduced oxygen concentration would decrease coke combustion rates, but the "ideal" regenerator could be made long enough to achieve any desired degree of coke removal. NOx formed, per unit of coke burned, would drop because NOx formation is strongly influenced by oxygen concentration.
Unfortunately, this "ideal" regeneration is not achievable in commercial catalytic cracking units, where 10-100 ton per minute of catalyst must flow as a fluid between a regenerator and a reactor, which are frequently at different elevations and may be at different pressures We wanted a practical way to achieve something which approaches the above ideal regeneration.
We discovered a way to achieve almost "ideal" regeneration which could be used in new units, or even retrofitted into existing regenerators. Our new process can use the shell of a conventional bubbling bed regenerator, and much of the equipment associated with it, to achieve efficient, generally co-current regeneration of FCC catalyst.